The STO can be called the most unSuccessful Token Offering method ever. It was used for a very short time and very few projects, and was not very popular. It was not very likely to be successful anyway.
Moreover, they were very cumbersome due to legal requirements.
I agree with you, they weren't very successful and not very popular, but I believe that's why we still need to keep an eye on. I mean, the ICO and IEO were very popular and there was a lot of hype around them and look at all those projects now. They are all dead, and most of them were a scam, just to grab the money from the communities. At least with the STO, you have some regulation and some real value behind the companies where you want to invest.
One thing I've learned from crypto is, that you shouldn't follow the masses and the hype. 99% of the time it's the wrong decision. IPOs have been around for ages and I don't see the same hype there was for ICOs/IEOs, but that doesn't mean that IPOs are not successful because they are not popular. The IPOs rule as of now, and I believe that in a few years, the STOs will have a spot with the big players.
Popularity and success have not much to do in this stage. It will come later